1. Field of the Invention
The present invention relates generally to a microwave oven, and more particularly, to an apparatus and method for controlling the output of a magnetron of a microwave oven, which is capable of stably obtaining a high value of output of the magnetron in the initial driving stage of the magnetron of the microwave oven.
2. Description of the Related Art
In general, microwave ovens are classified into two types: general microwave ovens and wall-mounted microwave ovens. The wall-mounted microwave ovens are called over-the-range microwave ovens and are installed over a gas range or a gas oven range in a kitchen.
The wall-mounted microwave oven functions not only to cook food but also to exhaust fumes, steam and the like that are generated by a gas range or a gas oven range disposed under the microwave oven.
In some countries where such wall-mounted microwave ovens are used, a maximum allowable current value of the microwave oven is limited. Accordingly, when a user desires to obtain maximum power of the magnetron of the wall-mounted microwave oven for cooking and/or exhaust operations, the maximum power should be limited to less than a certain value corresponding to the maximum allowable current value. Therefore, when the cooking and exhaust operations are carried out with the wall-mounted microwave oven simultaneously, a current that is applied to the microwave oven should be appropriately distributed between two operations (i.e., the cooking operation and the exhaust operation) so that a total value of applied current falls under the maximum allowable current value.
Where the maximum allowable current value of the microwave oven is limited, the rated current value of the microwave oven is generally limited to 15 amperes (A).
Accordingly, when the cooking and exhaust operations are carried out in the wall-mounted microwave oven simultaneously, the two operations are generally carried out within a rated value of 15 A.
A magnetron, which is a high-frequency generating device, can be controlled in an inverter manner. In accordance with the inverter manner, the output of a magnetron may be varied.
When normal heat generation is carried out with the magnetron, current, also known as anode current, passes through an anode of the magnetron and the magnetron generates high frequency output only when a heater of the microwave oven normally generates heat. A value obtained by multiplying the anode current by anode voltage, which is the voltage applied to the anode when the anode current passes through the anode, is power applied to the magnetron. In this case, the output power of the magnetron of the microwave oven is obtained by multiplying the applied power by an efficiency of the magnetron.
Accordingly, the output value of the magnetron is dependent upon the anode current. That is, as the anode current is increased, the output power of the magnetron is increased, and as the anode current is decreased, the output power of the magnetron is decreased.
With reference to FIGS. 1 and 2A–2D, a conventional method of controlling an output of a magnetron of a microwave oven is described.
FIG. 1 is a graph illustrating a characteristic of a general magnetron. FIG. 2A is a graph illustrating a conventional control of an anode current of the magnetron. FIG. 2B is a graph illustrating variations in an anode voltage of the magnetron according to the conventional control. FIG. 2C is a graph illustrating variations in an input current of the magnetron according to the conventional control. FIG. 2D is a graph illustrating variations in power consumption of the magnetron according to the conventional control.
As shown in FIG. 1, as time passes after the microwave oven is started, the characteristic curve of the anode voltage “V” of the magnetron is descending, while the characteristic curve of the anode current “A” is ascending.
The output of the magnetron of the microwave oven is controlled by adjusting the anode current. As shown in FIG. 2A, the anode current of the magnetron is controlled within an error range in a constant current control manner.
When the anode current is controlled in the constant current control manner as shown in FIG. 2A, the anode voltage descends and is saturated at a certain time as shown in FIG. 2B. The output of the magnetron descends in proportion to the anode voltage and is saturated at a certain time.
When the anode current of the magnetron is controlled in the constant current control manner with an initial input current of the microwave oven and the initial power consumption set, the anode voltage descends and is saturated at a certain time as described above and an input current and a power consumption over time are changed in proportion to the anode voltage as shown in FIGS. 2C and 2D. That is, as shown in FIGS. 2A, 2B, 2C and 2D, as the anode current is controlled in the constant current control manner, slopes of an input current curve and a power consumption curve become relatively low.
However, when the anode current is controlled in the constant current control manner to obtain a high value of output of the magnetron to ensure that electric parts, such as the magnetron, an inverter and the like, are not damaged in an initial driving stage of the microwave oven, the magnetron can obtain a high value of output increased by an extent corresponding to a difference between the initial input current and the saturated input current, thereby resulting in low efficiency of the microwave oven.
Additionally, as shown in FIGS. 2B and 2C, since the slopes of the anode voltage and input current become small, the initial input current value take a long time to be changed to the saturated current value.
For example, the anode current of the magnetron is controlled in the constant current control manner to produce a high value of output of the magnetron in the initial driving stage of the magnetron based on a condition that the initial input current value is set to 18 A and that the rated current value of the microwave oven is 15 A. In this case, the input current value take a long time to be changed to the rated current value of 15 A because the slope of the input current is small, so the conventional magnetron output control method is problematic in that electric parts, such as a magnetron, an inverter and the like, may be damaged by overheating.